See while you Measure: In-situ Studies in Mechanics (September Journal Club Topic)

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*Participation to this club is as easy as i) sending a reference/abstract of your paper if you are working in this area or ii) send a challenge concept/issue/expt if you are not experimentalist :)*

The September 2011 journal club theme is “In-situ Studies in Mechanics”. The basic concept is to perform the experiments under a microscope, so that the typical quantitative information (stress, strain, crack length to name a few obvious ones) is augmented with real time microscopy output. The advantages are many-fold; (i) one can ‘see’ the deformation and failure process to reduce intelligent guesses in modeling (ii) experimental/boundary condition accuracy is enhanced, particularly for nanoscale specimens, (iii) access to various domains typically not considered by the mechanics community. The power of visualization actually goes beyond these three salient features, a vicarious example of which is the Journal of Visualized Experiments (http://www.jove.com/). It is not really a mechanics journal, but the concept of expressing your thoughts and experiences with pictures and video boosts efficiency and liveliness in dissemination and learning instantly.

I am delighted to initiate this discussion by laying down few specific items that invite synergy as well as controversy. What I aim for this discussion is to bring up issues other than these, or at least to highlight the positives of in-situ studies, the associated ‘costs’ and whether it is worth doing as well as the future challenges. I have added a few references at the end of this posting for the zealous readers/participants. At the end of the month – I intend to follow up with a summary of your responses.

The Role Players: ‘In-situ Studies’ sounds like hardcore experimental research. However, the cream is rarely the experimental technique but the scientific finding. Perhaps the best players to stoke this area are the theoreticians/modelers who can challenge the experimentalists to support/refute their brain-childs.For example, dislocations were predicted and modeled long before they were seen – but the ability to see them made the transmission electron microscope the workhorse that it is. Being an experimentalist – I am eager to throw out the question: what are the outstanding issues in theory/modeling research that could be best addressed by in-situ studies? Or plainly describe a model or hypothesis of yours that you wish to be visualized in real time. What about other players – novel material synthesis, microscopy and miniaturization?

The Tools: Change the microscope and you change the mirror in the Kaleidoscope. Optical microscopy dominates the bio-materials research, but the low spatial resolution and surface-only probing might make it look less appealing for others. Yet, it could be the only tool to be equipped with really high speed cameras (and not to mention the largest possible work-volume to design the experimental setup). In-situ AFM type studies can provide premium resolution. On the other end, Transmission Electron microscopes (TEM) can visualize the internal features (dislocations, grain boundaries, voids – terms that directly relates to mechanics community) with up to atomic resolution. But the cost (not just monetary) has been pretty high. Not only drastic miniaturization is needed to measure anything useful but also the electron thin samples may show different behavior compared to the parent (exactly same bulk) material (analogous to forgetting to apply periodic boundary conditions in simulation?).

I am expecting a rich discussion on a plethora of issues including (i) whether or how important is the ‘real time microscopy’ for mechanics. Is conventional post mortem microscopy enough? Is in-situ straining (no stress or strain measurement) enough? (ii) is ‘seeing = believing’, or what else that we don’t see but is still there?, (iii)what are the undesired consequences of probing only very small volume of material? How can this be made up in the models (iv) what does the microscope itself do the specimen (irradiation/implantation)? (v) what are the challenges in sample preparation (such as, is the use of a focused ion beam safe?), (vi) how important it is to measure residual stress? (vii) how important is automated data collection? (viii) is there any way to improve the horrible temporal resolution of the visualization processes? I am certain there are many other concerns and I hope we are able tally and discuss as many as possible.

The Trades: Uniform tension, compression and nanoindentation are the two major types of experiments being performed. There is a remarkable lack in fracture, fatigue, creep as well as explicit interrogation of bi-material interfaces. Are there other pressing needs (for example – high or low temperature experiments?)

New Frontiers: Mechanics has seamlessly integrated phenomena that are non-mechanical in nature. Battery/Fuel cell/electro-chemistry; thermo-electrics/thermo-accoustics are just two (currently) high riding examples. However, in-situ studies on multi-domain are still rare. For example, in-situ STM or SThM studies could fundamentally advance our knowledge in strain dependence of electronic structure or thermal transport. I believe in-situ studies could gain a real cost-benefit edge by moving from tension/compression testing to multi-physics problems – where thermal, electrical, optical, magnetic, etc measurements could be made with the goal of tuning such with microstructure and deformation. For example, in-situ techniques can be particularly powerful for synthesis/growth reactor (example: http://www.youtube.com/watch?v=B099DRAX_X4&feature=related), or to study phase transformation.Another equally challenging (and rewarding) path is towards living cells, tissues and other bio-materials.

The list of papers below is incomplete, so please feel free to add to it.

Zhu, Y. and H.D. Espinosa, "An electromechanical material testing system for in situ electron microscopy and applications", Proceedings of the National Academy of Sciences of the United States of America, 2005. 102(41): p. 14503-14508.

Comments

Thanks so much Aman for posting such a hot experimental mechanics topic. The advancement in SEM/TEM has brought us the opportunity to truly see the sample and measure the force and displacement at the same time in situ. The papers listed are excellent for my graduate course as well.

Below please see our paper about in-situ SEM buckling of nanowires and calibration. In this work, the elastic modulus of single crystalline boron nanowires was measured by buckling individual nanowires with a nanomanipulator inside an SEM. A general calibration procedure for such in situ nanowire buckling testing was established through TEM cross-sectional imaging of the buckled nanowire to eliminate the relative error in determination of the nanowire cross section in the SEM.

Many thanks for making this post sticky. I read your paper with great curiosity (esp the unique buckling mode) when it came online. Quite challenging one - considering the sensitivity of the boundary conditions in buckling.

Thanks for discussing the in-situ SEM buckling of nanowires. I think buckling might be a viable testing method for nanowire mechanics in addition to other reported in-situ methods such as tension, bending, vibration and etc. As Aman pointed out, boundary condition is very important. Note, however, the buckling force is very small compared to tension. Clamping methods such as the electron-beam induced deposition have been used to break nanowires in tension. So we could quite safely assume that the fixed-fixed boundary condition holds if both ends are clamped.

On the other hand, buckling test is relatively easy to do in-situ (for instance, compared to three-point bending). So buckling might be a convenient method to probe the nanowire response under bending (nonuniform stress state). It is known that due to the surface effect, the nanowire elasticity is loading-mode dependent (e.g., tension versus bending). Indeed, we carried out both in-situ SEM tension and bucking tests, and found pronounced loading-mode dependence (in addition to the size dependence). See the paper below.

Tons of numerical simulations have been devoted to tension response of nanowires, though limited work on nanowire buckling including critical buckling stress (or strain), postbuckling and buckling-induced fracture.

like Chris already said, thank you very much for putting this hot topic together. Looking forward to a lively discussion which will for sure follow as you pointed on many tricky points.

In this sense, let me just briefly express my opinion on the issues you put up with in-situ:

(i) whether or how
important is the ‘real time microscopy’ for mechanics. Is conventional
post mortem microscopy enough? Is in-situ straining (no stress or strain
measurement) enough?

There are many things you can figure out by just comparing the before and after state. But there are more tricky examples (and every one of us in-situ guys has at least one demo video of such a case ;-) ) where you really need to 'see' what happens. In-situ straining is a valid technique for getting the general idea of what is going on, but why not going straight for a quantitative test if I can?

(ii) is ‘seeing = believing’, or what else that we
don’t see but is still there?

Seeing makes things easier, but don't believe blindly just because you see something, as it might be simply an artefact. Being an electron microscopy guy like Aman, we are of course aware that there are limitations. In the SEM, you only see the surface, what ever happens in the sample volume remains a matter of scientific creativity and post characterization. In the TEM, you can see 'everything'...but only if it is visible. Extinction conditions will hide a certain fraction of the features (e.g. dislocations). Or the action might just happen few nm outside of your imaged area. Here the quantitative testing comes in handy: If you see something happen, and you can correlate it to your mechanical data, then you can reasonably safe assume that you cought the essential part, whatever happened with the invisible features or outside your field of view. If you see nothing structurally changing while the sample hardens, softens, ...., then you miss the important things as they are out of contrast or happening somewhere else.

(iii)what are the
undesired consequences of probing only very small volume of material?
How can this be made up in the models

Scatter. The stochastics don't improve when the samples become every smaller. Does my volume contain a dislocation, or two? Or maybe none and deforms at the elastic limit? (1) Also, we showed recently that the yield strength of irradiated Cu can be measured from miniaturized specimens, but exhibits some scatter (2). We will need better statistical models to handle these issues. And of course the surface fraction increases, leading straight to the next two points.

(iv) what does the microscope
itself do the specimen (irradiation/implantation)?

This strongly depends on your material. For metals, we feel rather safe. But with the upcoming environmental TEMs and the move towards biological materials, this will become more of an issue. Vaccuum, low temperatures, high intensity electron beam...not every material sustains such conditions without problems. Just think of the early graphene observations. Essentially, these people were shooting holes in the sample and watched them grow. Once you reduce your voltage below the knock off threshhold (3), you can image the sample nearly forever and get atomic information (4).

(v) what are the
challenges in sample preparation (such as, is the use of a focused ion
beam safe?),

FIB is like many preparation methods not safe in general. You create numberous defects in your near surface structure (5). So if you are interested in studying pristine volumes, you need to e.g. heal your sample afterwards (1). If your specimen contains many internal defects (2), the FIB damage will be of minor concern. So, the FIB is very convenient, but one should keep it's possible material modifications in mind.

(vi) how important it is to measure residual stress? (vii)
how important is automated data collection?

Tons of thin film literature tell us that residual stresses are important. And there are very elegant emerging methods for local residual stress (strain) measurement (6).

(viii) is there any way to
improve the horrible temporal resolution of the visualization processes?

This is the worst thing about in-situ testing. I get thousands of data points per second, but only 33 fps or so. If you have bad luck, the action happens just in between two video frames. But there are emerging new detectors (7) that can operate with several 100 fps. Not sure if they are commercial so far, but they will change our way of viewing.

The references are of course just a selection and by no means comprehensive:

Lively discussion starts with you Dan! Many thanks for adding the references.

1. No doubt that numbers and pictures together maximize the gain. While I go for that myself, often I get tempted to run in-situ straining experiments because of the simplicity.

2. I predict that "Seeing = believing" will be the most discussed section. I believe all of us check the dislocation invisibility, beam heating etc routine checks. Yet some times we face questions that are answered right there in the supplementary video.

3. Very good point on the small probe volume. On a different thought - I like nanoindentation (even though I dont do it) better than tensile because the small probed volume concides with where all the action is. Torsion would have been really good for this purpose too.

4 & 5. Yup, this is another hot bed. The more we discuss on this - more careful we can be.

7. I have been reading papers with next generation TEM concepts and progresses. Quite itching to see these realized and available.

Thanks Aman for raising the timely issue. Thanks also to Dan and Xiaodong for very fruitful comments. Dan, I cannot agree more with your views. Particularly regarding the point IV. Unless a safe electron energy is established for a given material, and carefully verified, one can get many misleading observations from both SEM nd TEM. I hope, existing literature does not have too many of such results.Regarding point I, Is in situ straining enough? I.e., no stress measurement. I guess, the question is, can we really measure stress at the region of interest where some mechanism is getting activated. We can of course measure the macroscopic stress which has somethibg to do with the local stress. But unless the far field macroscopic stress to local stress mapping is known (such as, far field stress to local crack tip stress in linear elastic fracture mechanics), local stress remains unknown. It is this local stress that is driving the local mechanisms. So, in situ stress-strain measurement is still a utopia.

So far, the in situ tests have been limited to observing dislocations and their motions. I wander, is there anything in the horizon to "see" dynamics of other defects.

This is indeed one of the biggest challenges in making the most out of an in-situ test. All we are doing so far is - 'measuring globally applied stress/strain while seeing the small volume of material' (probed by the electron beam). Or in other words, seeing a dislocation or GB moving under a measureable far field stress itself remains a big challenge now - let alone local stress-strain measurement.

The better news is that a reasonable estimate of local strain can be obtained through selected area electron diffraction. Here, the diameter of the diffraction ring patterns are related to the interatomic distance for that specific xtallographic plane. So, strain can be accurately measure by comparing the diameters of stressed and unstressed states. Not so good news is - the spatial resolution is not the best - so we get averaged values over tens of nanometers (fairly localized)

In the figure below, we show the two diffraction patterns - one at a notch tip and the other far away from the notch. This was an evidence we used in a recent paper where we argued that the concept of stress concentration may not be applied readily to nanocrystalline metals where GNDs are absent (and possibly replaced by GB motion).

Thanks for initiating this interesting discussion and posing several well-thought-out questions. I share Dan and Taher’s view that we must understand what microscope does to the samples (e.g., possible sample heating, effect on defect mobility and etc). In addition, I came across a recent review article by Ian Robertson and others. It provides an excellent summary and future directions of a broad spectrum of material characterization toolbox (not only SEM/TEM, but also x-ray tomography, neutron tomography, atom probe tomography and others). I believe many of these tools can enhance our capability of “see while you measure”.

Thanks for the interesting discussion! I'd like to point out a few things regarding in-situ TEM, based on my own experience.

1. Let's be more optimistic about in-situ SEM/TEM, a lot of microscopists think that in-situ TEM is the future of electron microscopy. Abberation Correction technology is already there, which makes SEM/TEM even more powerful than before in terms of spatial and energy resolution. Now several groups are working on low voltage AC-TEM with an accerelating voltage as low as 30 kV, yet still has sub-Angstronm resolution, the low voltage TEM will greatly reduce the knock-on damage in samples.

2. A lot of times by doing a "blank beam" experiment to compare with the beam-on experiments, you'll get a pretty good idea about how much beam influnence you are having. We are working on in-situ lithium ion battery (LIB) studies, and it is well known Li is very sensitive to electron beam, and a lot of battery people ask us how much beam influence on our experiments. We've done beam blank experiments, which showed similar results to the beam-on results. But battery people still question the relevance of in-situ battery to real battery system, since the experimental configurations between the two are so different. Fortunately we can compare the cycling results between the two, and the results are very promising. For example, we recently predicted that Ge is a better anode than Si in terms of charging rate and cyclability from our in-situ studies, and similar conclusion was reached by indpedent studies conducted on conventional electrochmical cell, which boost our confidence on our in-situ studies. In this case our in-situ studies provide insight into why Ge has better performance than Si, and such insight can not be obtained by any other means. We have many this kind of examples.

3. Reagarding the in-situ mechanical test, a key question is: can we do more quantitative rather than just qulitative measurements? Can we have single dislocation sensitivity in the stress-strain plot? These appear to be instrumental limited.

4. Beam heating effect: I would say for metals this should not be significant. We put Ga (melting T ~30 oC) into our TEM, and under normal imaging condition, we do not see melting of Ga, meaning the beam heating should be less than 30 oC. But like Taher said, this is material dependent.

In summary, i'd say that in-situ TEM has a big future, with advancement in electron microcopy technology and the availability of all kinds of in-situ holders, such as mechanical, biasing, optical, liquid, gas holders, the capability of TEM is greatly extended, TEM has evolved from a traditional structural charization tool to nano or pico measurement machines. Atomic structure and electrical, mechanical, thermal, optical, magnetic, and electrochemical properties can all be measured in a TEM.

1. The goal for higher spatial and temporal resolution is unfortunately on the technical developers hand, and we the common users can do little but wait for the technology to be here and feasible. One example is having a high speed camera in a TEM. So we patiently wait for access to one.

2. Excellent point on blank beam experiments ; I think this is a great advice for starters in this area. Actually - this also goes a long way in touting semi-quantitative experiments. I agree with all of you that quantitative is best but qualitative is better than nothing. A blank beam experiment will give the data but wont show anything, and with the beam on, we will see something (and the rest depends on how intelligently we can analyze the qualittaive information and come up with physical or math models)

3. "Single dislocation sensitivity" - as Taher points out, it belongs to utopia :) I think it is possible to measure stress so small that single dislocation can be manipulated - but the problem is that the stress measurement is so far only global .

I really appreciate your summary statement. I too believe, all of it will come true. This might have been accelerated if the equipment or access were cheaper. My hobby to develop tools that measure all thermo-physical properties. But the big challenge there is the cost of holder development - far away from a single PI reach

This is a great topic, and I wish I were a biomaterials expert! My understanding is that to some extent - vaccum is far away from the native state and this will be reflected in the in-situ TEM data. You are right - the difference in states could be smaller for materials such as bone, teeth and very significant for softer materials such as cells and tissues. It is very possible to perform tests in materials for which hydrated states are not necessary such as silk fibers (I did one in SEM). I also performed in-situ testing on an onion cell sample - which shows very high stiffness (3 GPa) when dry but I am certain this would change when hydrated.

My feeling is with the advent of liquid cell holders, even hydrated experiments can be performed. Not with the off the self liquid cell holders - but after augmenting that with micro sensors and actuators. Unfortunately the space is not much but I believe it is possible. nevertheless, this is an area where in-situ probe microscopy can be quite useful.

Most of the papers on TEM relate to truly engg materials with bio application so I dont brand them as bio materials in a conventional way. Will have to get back to you on a worthwhile reference paper.

Thanks Majid and Aman. There are only few papers on in situ TEM studies on biomaterials. Recently, my group (Dr. Jianfeng Zang) used SEM to perform tensile tests on rod-like virus-based composite nanofibers, see the paper listed below. I think that in-situ TEM will give much more insights into the correlation between structure change and deformation behavior.

In many cases, people used EBID to deposit carbon to bond the nanowire to the tensile stages. The mechanical properties of EBID carbon are of importance for the in situ testing. The following paper may help to know how much the bond contributes to the test results.

Thanks for initating this topic. We have also started to look on the size effect and elactic properties of materials used in fuel cells (since the layes are <20 micron thick/thin).

I would like to get an insight from everyone on the specimen (tension) preparation. As we know, one of the major issue with FIB is it is not a cheap and quick solution. One of the fix which could be the use of high intensity low enerygy Argon to fabricate the specimens which could potentially lead to no's of specimens (geometry depend on the type of mask) but are there any major disadvantages to it ?

FIB in sample preparation is most discussed topic and the verdict is: cant live without it and cant live with it (Like delhi-ka laddoo). I am not quite familar with the high intensity lor energy Ar technique. can you please post a reference for everyones convenience? Looking forward to it.

We performed an electromigration study inside the TEM, only the sample was not exactly like a conventional one. Instead of completely confined by substrate/passivation, our sample was freestanding, so that does a lot by changing the boundary condition. nevertheless. we could still measure local stresses at the anode and cathode to confirm the stress due to electron wind. the temperature profile in the sample became parabolic because the sample is freestanding. What we saw:

4. Further increase in current density led to fracture of the specimen at 6x10^6 A/cm^_2

One single experiment allows us to map the deformation mechanisms as a function of current density/mechanical stress. Another way to view it is - single sample will allow us to study mechanical property for wide variety of grain sizes.

As lamented earlier, the poor temporal resolution of TEM doesnt make in-situ fatigue studies appealing. I mean we can do the experiment at say 10 kHz, but cant see a darn thing. So we performed from 2 to 10 Hz studies on freestanding aluminum.

The sample never failed, so we cut a notch almost half the width (if there is stress concentration, it will lead to about 5 GPa stress at the notch). yet after several millions cycles the sample didnt fail. What we propose is that dislocations contunue to escape through the surface of the electron transparent films, and the grain rotations (marked by contrast changes in the video below) are reversible. There is no chance for persistent slip bands, so no fatigue crack initiation. The sample actually never failed but at $40/hr TEM charge it dinged me (not that I am very unhappy)

The easiest way we can make this discussion prolific is to refer to a paper that you have found interesting (preferably your recent work or even not necessarily yours). Or, if you are not an experimentalist - to throw a challenge to the experimentalists that will stimulate them.

Thank you for posting such an interesting topic and leading the thread. It seems the discussion above is mainly focused on in-situ SEM and TEM, which are powerful tools for exploring mechanics of "hard materials" (e.g. metal, ceramics). I would like to share our experience with in-situ mechanics of "soft materials" (e.g. polymers, gel, cells) with optical microscopy. As you mentioned, the flexibility and large working volume of optical microscope enabled us to couple the microscope with other facilities such as high electric/magnetic field generators. The system can be used to explore a variety of multi-physics problems on the interface between mechanics and other disciplines. One example is an in-situ observation of deformation and intabilities of electroactive polymers.

Oh - very COOL!! Thanks for sharing this. Special thanks for letting the discussion move beyong TEM or SEM. I sincerely hope to other applications (optical, AFM, Raman, Xray etc). I am going to read this paper this weekend.

Hi Aman,thank you for sharing your ideas about in-situ studies in Mechanics.Since my research area is experiamental machanics,I'd like to say that in-situ sutdies of mechanics is so powerful that it could provide us plentiful information about the deformation and structure evolution of the materials.I think we share the same concept about the in-situ studies,namely synergistic measurement. Combined with SEM/TEM techniques,in-situ Raman test can give us lots information about the deformation process.we are now working on it.I'd like to share our experiment after finishing it.

Raman appears to be high potential yet least utilized too for in-situ testing. May be because it appeals more to Chem E people than mechanics. I believe that it will appear to be indispensible for multi-physics type testing. The figure (taken from web - not mine) below shows why.

Thanks, Aman, for providing the raman picture. It is true that raman test is a very popular tool appealing to many researchers in Chem/Physics field. Nevertheless, it does reveal the internal structure of the materials, which has connection with stress/strain in microscale. From the picture, you see, the raman test can tell us such plentiful and favorable stories about the testing materials as frequency, width and intensity. Hrere is a typical research of our group:http://iopscience.iop.org/0957-4484/22/22/225704.

BTW, I am also very interrsted in TEM, and have learned a lot from the pretty nice discussion. I am going to study on it. Thank you again.

Thanks, Aman, for opening a nice discussion with many thoughtful questions.

Most of the discussion above concerns in-situ observation using TEM. I will comment on in-situ observations of fracture in SEM. SEM observations do not provide resolutions to see dislocations as TEMs do, but are still very useful for the study of microscale failure mechanics in advanced materials such as fibre composites.

We have developed a number of loading devices for mechanical testing in an ESEM (Environmental scanning electron microscope). An ESEM is useful since the specimen surface does not need to be conducting. Therefore, newly cracked surfaces can be imaged since they do not charge-up.

ESEM testing is in many ways easier (I think) than testing in TEM. Specimen dimensions (also thickness) can be in the order of mm or cm, so that specimen preparation is not very difficult. The relative larger specimen dimensions make it easier to develop various loading devices that can perform advanced and well analysed mechanical testing experiments. For instance, we have developed loading devices for perform tensile testing, compression, 3-point and 4-point flexures as well as stable Mode I and mixed mode cracking experiments to go into the vacuum chamber of the ESEM.

For fracture experiments, one of the challenges is to develop loading devices that enables stable crack growth. For instance, for Mode I we use DCB (double cantilever beam) specimens loaded with pure bending moments - not transverse forces like the standard DCB.

Many thanks for sharing the references and I am sorry if the general impression is that this Journal club is TEM oriented. SEM has been a workhorse for mechanicians - and indeed, ESEM is a technique better suited for materials that do not conduct well (I think bio materials would be appropriate as well).

The term Multi-physics is getting more attention from mechanics and physics of materials perspectives. It is a multi-scale phenomenon but my personal focus is the coupling among mechanical, electrical, thermal etc domains "due to the length-scale" (especially for materials such as metals, which show weak or no coupling at the macro scale. Because specimen size does this trick, in-situ testing may point out what aspect of size or deformation behavior is responsible for thermal conductivity or electrical resistivity changes in metals.

The experiments (in or ex situ) very hard because not only mechanical testing in nanoscale is challenging, one has to pay meticulous attention to the electrical and thermal boundary conditions for multi-physics type testing. I expect to see more work in this area, and your thoughts on this topic.

Another challenge problem could be high temperature (>1000 C) studies on materials behavior insitu. It is very hard to 'inject' the heat in the sample, if the cross-section is nanoscale. On the other hand, localized heating (a must for in-situ chambers) is is difficult for larger cross-sections. The literature has MEMS heaters - but to integrate that with specimens and force/displacement sensors is yet to be seen. One not so elegant way for metallic sample is resistive heating. The following ultra-low mag TEM video (wont show you GB or flaws) shows the role of electromigration, temperature, stress etc on the failure mode.

Another challenge is to perform in situ tests in harsh environments. For instance, the effect of water molecules on the mechanical properties of nanostructures. SEM and TEM need vacuum. But for AFM, it is not required. AFM can work in the open air. My group (Yingchao Yang, PhD student) in collaboration with Prof. Guofeng Wang used this novel in situ AFM mechanical testing technique. We found that water molecule can induce stiffening in ZnO nanobelts. For details, please see the following paper.